nanomaterials Article
Plasmonic Sensor Based on Interaction between Silver Nanoparticles and Ni2+ or Co2+ in Water Federico Mochi 1,2 , Luca Burratti 1 , Ilaria Fratoddi 3 ID , Iole Venditti 4, * ID , Chiara Battocchio 4 Laura Carlini 4 , Giovanna Iucci 4 ID , Mauro Casalboni 1,2 , Fabio De Matteis 1,2 , Stefano Casciardi 5 , Silvia Nappini 6 ID , Igor Pis 7 ID and Paolo Prosposito 1,2, * ID 1
2 3 4 5 6 7
*
ID
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Department of Industrial Engineering and INSTM, University of Rome, Tor Vergata, via del Politecnico 1, 00133 Rome, Italy;
[email protected] (F.M.);
[email protected] (L.B.);
[email protected] (M.C.);
[email protected] (F.D.M.) Center for Regenerative Medicine, University of Rome Tor Vergata, Via Montpellier 1, 00133 Rome, Italy Department of Chemistry, University of Rome Sapienza, Rome, P.le A. Moro 5, 00187 Rome, Italy;
[email protected] Department of Sciences, Roma Tre University of Rome Via della Vasca Navale 79, 00146 Rome, Italy;
[email protected] (C.B.);
[email protected] (L.C.);
[email protected] (G.I.) National Institute for Insurance against Accidents at Work (INAIL), Department of Occupational and Environmental Medicine, Epidemiology and Hygiene, 00078 Monte Porzio Catone, Italy;
[email protected] IOM-CNR Laboratorio TASC, SS 14, km 163,5 Basovizza, I-34149 Trieste, Italy;
[email protected] Elettra-Sincrotrone Trieste S.C.p.A., SS 14, km 163.5 Basovizza, I-34149 Trieste, Italy;
[email protected] Correspondence:
[email protected] (I.V.);
[email protected] (P.P.); Tel.: +39-06-5733-3388 (I.V.); +39-06-7259-4115 (P.P.)
Received: 28 May 2018; Accepted: 28 June 2018; Published: 2 July 2018
Abstract: Silver nanoparticles capped with 3-mercapto-1propanesulfonic acid sodium salt (AgNPs-3MPS), able to interact with Ni2+ or Co2+ , have been prepared to detect these heavy metal ions in water. This system works as an optical sensor and it is based on the change of the intensity and shape of optical absorption peak due to the surface plasmon resonance (SPR) when the AgNPs-3MPS are in presence of metals ions in a water solution. We obtain a specific sensitivity to Ni2+ and Co2+ up to 500 ppb (part per billion). For a concentration of 1 ppm (part per million), the change in the optical absorption is strong enough to produce a colorimetric effect on the solution, easily visible with the naked eye. In addition to the UV-VIS characterizations, morphological and dimensional studies were carried out by transmission electron microscopy (TEM). Moreover, the systems were investigated by means of dynamic light scattering (DLS), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and high-resolution X-ray photoelectron spectroscopy (HR-XPS). On the basis of the results, the mechanism responsible for the AgNPs-3MPS interaction with Ni2+ and Co2+ (in the range of 0.5–2.0 ppm) looks like based on the coordination compounds formation. Keywords: silver nanoparticles; surface plasmon resonance; heavy metal ions sensing; Ni2+ sensing; Co2+ sensing; water pollution; optical sensors
1. Introduction The huge development of materials science, nanoscience, and nanomaterials technology has led to the synthesis and engineerization of several nanostructures (metallic and non) used in different fields such as biomedicine [1,2], biotechnology [3,4], energy [5–8], optics, and optoelectronics [9–16]. Recently, sensors based on different nanosized materials have been developed, achieving a high sensitivity and selectivity [17–26]. Nanomaterials 2018, 8, 488; doi:10.3390/nano8070488
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In this framework, metallic nanoparticles cover an important role for the easy synthesis, the low costs, and for the possibility to accomplish specific external functionalization to respond selectively to specific analytes. The reduced dimensions of the metallic structures confer them unique physical, chemical, mechanical, optical, magnetic, and catalytic properties [27–32]. In particular, the small dimensions allow optical properties, which make metallic nanoparticles interesting for optical spectroscopy applications such as surface enhanced Raman spectroscopy (SERS) and surface plasmon resonance (SPR). The latter phenomenon occurs when an electromagnetic radiation of a certain wavelength, exciting the nanoparticle, causes the oscillation of free electrons in the conductive band. As a result, the optical absorption presents an intense and well-shaped SPR, usually in the visible region. The wavelength of the SPR peak strongly depends on the type of metal, particle dimension, shape, and chemical environment. Another important aspect of these systems is their high surface to volume ratio, which confers them a very high reactivity with the surroundings as well as the possibility to modify their external surfaces with an appropriate surface chemistry. On this basis, many metal nanoparticles-based sensors have been studied and developed [33–37]. The main application of such optical sensors is the detection of heavy-metals, which have long been known to be harmful for the environment and toxic for human health, above very small concentrations, a few ppm or lower, as reported in the literature [38–40] and by the Guidelines for Drinking-Water Quality by the World Health Organization (WHO) [41]. The current state of the art for the detection of heavy metal ions is based on complex and time consuming techniques, such as high performance liquid chromatography (HPLC), atomic fluorescence spectroscopy (AFS), flame atomic absorption spectroscopy (FAAS), and graphite furnace atomic absorption spectroscopy (GFAAS) [42,43]. All of these methods are very sensitive and reliable, but they have also some disadvantages, as the complexity and the high instrumentation costs, together with the requirement of highly skilled operators. For all these reasons, the scientific community is currently working on innovative, simple, and low cost heavy metal ions sensors. In particular, the ones based on optical and colorimetric techniques have received great attention, as they can offer high selectivity, stability, intrinsic operational simplicity, and immunity against electrical disturbance. In addition, they are extremely attractive as they are based on simple and low cost materials, are very easy to use, are portable, and need simple and cheap set ups, offering, at the same time, high sensitivity and selectivity. In the present work, AgNPs-3MPS were synthetized and their interaction with heavy metal ions was studied using different techniques. The system presents a strong sensitivity to Ni2+ and Co2+ ions, showing a consistent change in the SPR as a function of the ion concentration, resulting in a colorimetric change of the solution. The morphological and dimensional characterizations of the AgNPs-3MPS (average size and shape) before and after the interaction with Ni2+ and Co2+ were obtained by TEM studies. Moreover, the system was studied by means of different techniques, such as dynamic light scattering (DLS), UV-VIS, FTIR, and high-resolution X-ray photoelectron spectroscopy (HR-XPS), in order to understand the mechanism of SPR sensing. 2. Materials and Methods 2.1. Materials Silver nitrate (AgNO3 , 99.5%, Sigma-Aldrich, St. Louis, MO, USA) and sodium borohydride (NaBH4 , 98%, Sigma-Aldrich, St. Louis, MO, USA), were used for the synthesis of the nanoparticles. 3-mercapto-1propanesulfonic acid sodium salt (C3 H7 S2 O3 Na, 3MPS, Sigma Aldrich, 98%) was used as a capping agent. For the sensitivity measurement to the different ions, we used the following salts: Mg(ClO4 )2 , KClO4 , NaClO4 , Ca(ClO4 )2 , Pb(NO3 )2 , Cd(NO3 )2 , FeCl3 6H2 O, Cu(NO3 )2 , NiCl2 6H2 O, and CoCl2 6H2 O. For all of the solutions, we used deionized water (electrical conductivity less than 1 µΩ/cm at room temperature) obtained from a Millipore Milli-Q water purification system. All of the reagents were purchased from Sigma Aldrich and were used without further purification.
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2.2. Synthesis and Characterization of AgNPs The AgNPs were prepared by a wet reduction of silver nitrate in the presence of sodium borohydride. The thiol (3MPS) was subsequently added and it capped the silver nanoparticles. The details of this procedure were reported elsewhere [44]. The morphological characterization was accomplished with a TEM, FEI TECNAI 12 G2 (120 KeV) apparatus, equipped with an energy filter (GATAN GIF model) and a Peltier cooled SSC (slow scan charged coupled device) multiscan camera (794 IF model). A droplet of AgNPs water solution was placed on a copper TEM grid (mesh 400) coated with ultrathin carbon support. The UV-VIS spectra of water suspensions were collected using a Perkin-Elmer Lambda 19 spectrophotometer. The DLS measurements on the AgNPs colloidal suspensions (0.2 mg/mL) at T = 25.0 ± 0.2 ◦ C were performed by the Malvern Zetasizer Nanoseries instrument (Malvern, UK), as reported in previous studies [45]. The ζ-potential was calculated from the measured electrophoretic mobility by means of the Smolukovsky equation [46]. Reflection absorption infrared spectroscopy (RAIRS) analysis was performed by means of a VECTOR 22 (Bruker, Billerica, MA, USA) FTIR interferometer equipped with a deuterated-triglycine sulfate detector (DTGS detector), and operating in the wavenumber range 400–4000 cm−1 . The measurements were carried out by means of a Specac Monolayer/grazing angle accessory GS19650, operating at 70◦ incidence. The samples were prepared as thin films by solvent evaporation on Ti substrates from the mother solution; a clean Ti surface was used to record the background. The HR-XPS experiments were carried out at the BACH (Beamline for Advanced DiCHroism) line at the ELETTRA synchrotron facility in Trieste (Italy) [47], to probe the nature of the interactions at the AgNPs/organic ligands interface and the formation of Ni2+ or Co2+ coordination compounds. All of the samples were deposited by means of a drop casting procedure on a silicon wafer substrate (TiO2 /Si (111 plane)). The XPS data were collected in a fixed analyzer transmission mode (pass energy = 30 eV). Photon energies (PE) of 380 eV were used for C1s and S2p spectral regions, with an energy resolution ∆E = 0.2 eV, and a PE of 1050 eV was selected to acquire Ag3d, O1s, Ni2p, and Co2p core levels spectra, with an energy resolution ∆E = 0.3 eV. The aliphatic C1s signal and metallic Ag3d5/2 signals were used for the energy scale calibration. The XPS data analysis was performed via the curve-fitting of S2p, Ag3d, Ni2p, and Co2p experimental spectra, using a combination of Voigt shaped peaks, after the subtraction of a Shirley background. The S2p3/2-S2p1/2, Ag3d5/2 -Ag3d3/2 , Ni2p3/2 -Ni2p1/2 , and Co2p3/2 -Co2p1/2 doublets were fitted using the same full width half maximum (FWHM) for the two spin-orbit components of the same signal, a spin-orbit splitting of 1.20 eV for S2p, 6.00 eV for Ag3d, 3.30 eV, 17.27 eV for Ni2p, and 14.97 eV for Co2p, and the branching ratios S2p3/2 /S2p1/2 = 2, Ag3d5/2 /Ag3d3/2 = 3/2, Ni2p3/2 /Ni2p1/2 = 2, Co2p3/2 /Co2p1/2 = 2 have been detected, respectively. For the S2p XPS spectra, many chemically different species of the same element were identified and the same FWHM value was used for all of the individual photoemission bands, in order to reduce the number of refinement parameters, and then improving the reliability of the results. In the Co2p and Ni2p spectral regions, shake-up satellites appear nearby the main photoelectron signals (at higher binding energy (BE)), as expected for the transition metals ions [48]; to fit satellite signals, variable FWHM and branching ratios were used, accordingly to the literature [49,50]. 2.3. Sensing The AgNPs-3MPS contained in a fixed volume of water (typically 0.014 mg in 1 mL) were added to a fixed volume of water solution containing the heavy metal ions at specific concentration (typically in 1 mL). After five minutes of interaction of the nanoparticles with the metal ions, the optical absorption spectra and the respective DLS measurements were collected. The response to several metal ions was tested by UV-VIS spectroscopy.
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3. Results and Discussion Figure 1 reports the SPR absorption band of the AgNPs-3MPS (reference solution) and the optical features of the same solution with a different concentration of nickel and cobalt ions, up to Nanomaterials 2018, (a) 8, x FOR REVIEW 4 of 14 2.0 ppm for nickel andPEER up to 3.0 ppm for cobalt (b), respectively. A red shift with the increasing concentration of ions is observed for both of the ions as well as a broadening of the SPR band. ppm for nickel (a) and up to 3.0 ppm for cobalt (b), respectively. A red shift with the increasing Figure 1c,d show the wavelength shifts (∆λ) and the variation of the full width at half maximum concentration of ions is observed for both of the ions as well as a broadening of the SPR band. Figure (∆FWHM) of the Ag absorption band for increasing Ni2+ and Co2+ concentrations. We decided to 1c,d show the wavelength shifts (Δλ) and the variation of the full width at half maximum (ΔFWHM) report of theband absorption band, Ni as2+ itand gives qualitative idea the broadening of the of the the FWHM Ag absorption for increasing Co2+aconcentrations. We of decided to report the nanoparticles dimension distribution before and after the interaction with ions. Both of the figures FWHM of the absorption band, as it gives a qualitative idea of the broadening of the nanoparticles showdimension a saturation effect, represented thethe plateau for high ionions. concentrations, ppm for Ni2+ distribution before andby after interaction with Both of the 2figures show a and 2+ 2+ representedeffect by the ion concentrations, 2 ppm for for nickel Ni and 3ppm 3ppmsaturation for Co .effect, The saturation is plateau reachedforathigh different ions concentration and cobalt. 2+. The saturation effect for Co is reached at different ions concentration formodification nickel and cobalt. The interaction of the Ni2+ ions with AgNP-3MPS causes a stronger of the shape and 2+ ions with AgNP-3MPS causes 2+ The interaction of the Ni a stronger modification of for the 2.0 shape wavelength peak of the SPR with respect to presence of Co . In particular, ∆λ is 36 nm ppm of and wavelength peak of the SPR with respect to presence of Co2+. In particular, Δλ is 36 nm for 2.0 2+ 2+ Ni , while only 17 nm for 3.0 ppm of Co . A similar behavior was found for ∆FWHM, 67 nm for ppm of Ni2+, while only 17 nm for 3.0 ppm of Co2+. A similar behavior was found for ΔFWHM, 67 nm 2.0 ppm of nickel and 36 nm for 3.0 ppm of cobalt. Fitting those curves with a sigmoidal Richards for 2.0 ppm of nickel and 36 nm for 3.0 ppm of cobalt. Fitting those curves with a sigmoidal Richards function (y = a × (1 + (d − 1) × exp(−k × (x − xc)))(1/(1−d)) ), it is possible to obtain a correlation function (y = a × (1 + (d − 1) × exp(−k × (x − xc)))(1/(1−d))), it is possible to obtain a correlation between Δλ between ∆λ and ∆FWHM with concentration. The fittingofparameters the function varies (see for each and ΔFWHM with concentration. The fitting parameters the functionof varies for each system system (see supporting information Figure S1graphics for fitting and table of parameters). supporting information Figure S1 for fitting andgraphics table of parameters).
Figure 1. Optical absorption bands for Ni2+ (a) and Co2+ (b) as a function of the ion concentration
Figure 1. Optical absorption bands for Ni2+ (a) and Co2+ (b) as a function of the ion concentration (reported in the figure labels); variation of Δλ and variation of the full width at half maximum (reported in the figure labels); variation of ∆λ and variation of the full width at half maximum (∆FWHM) (ΔFWHM) of Ni2+ (c) and Co2+ (d), as a function of the ion concentration. of Ni2+ (c) and Co2+ (d), as a function of the ion concentration.
We checked the selectivity of the colloidal system to other ions. Figure 2 (upper part) shows the variation of the plasmonic (peaktowavelength shape) of the AgNP-3MPS We checked theoptical selectivity of thecharacteristics colloidal system other ions.and Figure 2 (upper part) shows the solution withoptical 1.0 ppm of the specific ions listed in (peak the figure. The response the non-toxic such variation of the plasmonic characteristics wavelength andtoshape) of the ions AgNP-3MPS 2+, K+, Na+, and Ca2+, and to the toxic agents such as, Pb2+, Cd2+, and Fe3+, is clearly not as Mg solution with 1.0 ppm of the specific ions listed in the figure. The response to the non-toxic ions shows a small of the shape (ΔFWHM) of2+the SPR negligible + ,2+Na + , and 3+ , is clearly such significant. as Mg2+ , KCu Ca2+ ,modification and to the toxic agents such as, Pb , Cd2+ , and andaFe change of the maximum peak wavelength. The most relevant differences were measured for Ni2+ not significant. Cu2+ shows a small modification of the shape (∆FWHM) of the SPR and a negligible 2+ and Co . Figure 2 (lower part) reports a picture of the AgNPs-3MPS solution treated with a fixed change of the maximum peak wavelength. The most relevant differences were measured for Ni2+ amount (1 ppm) of different ions; the colorimetric changes can be easily appreciated also by naked
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and Co2+ . Figure 2 (lower part) reports a picture of the AgNPs-3MPS solution treated with a fixed Nanomaterials 8, x FOR PEER REVIEW 5 of 14 eye. amount (1 ppm)2018, of different ions; the colorimetric changes can be easily appreciated also by naked The different optical response to nickel and cobalt ions can be exploited for a selective detection of eye. The different optical response to nickel and cobalt ions can be exploited for a selective detection theseof specific species. these specific species.
Figure 2. Δλ and ΔFWHM of the AgNPs-3MPS solution with 1 ppm of different metal ions. Image of
Figure ∆λ and ∆FWHM the AgNPs-3MPS solution with 1 ppm of different metal ions. Image of the2.colorimetric aspect ofof1.0 ppm solution of different ions. the colorimetric aspect of 1.0 ppm solution of different ions.
The DLS measurements were carried out, showing an increase of the average hydrodynamic diameter H>) of the AgNPs-3MPS, after interaction with the Co2+ or Ni2+ ions (see supporting The DLS (
